Why are some parts of the body hairy and others not?

Can anyone explain to me the evolution of human hair? Why have some areas of our body lost their hair? Why does the hair on our head continually grow longer unlike other hair? and why do men have hair on their face but women do not? Thanks

another_someone

quote:Originally posted by MelissaCan anyone explain to me the evolution of human hair? Why have some areas of our body lost their hair? Why does the hair on our head continually grow longer unlike other hair? and why do men have hair on their face but women do not?

I think the simple answer is that people do not really know the answer. When no-one knows the answer, then lots of answers get guessed at, but no-one knows which is the right guess.

There are many aspects of the human body that are equally inexplicable, such as why women have large breasts (many other female mammals have nipples but no external breasts).

The simplest guess is sexual selection. That early males and females thought people with these features were more attractive, so people with these features had more children, and so more off their offspring had these features, until it came to a point that all humans had these features.

One guess I have heard concerns human eyesight (in fact the eyesight of all primates). The human eye is not equally sensitive to all colours, and one theory has it that the red and blue sensitivity of the human eye is clustered closer together to give it greater sensitivity to the colour of blood. In particular, that the human eye is sensitive to whether people are blushing, or the skin is blanched with fear, etc. - but all of this presupposes that there must be visible skin, such as on the face, that humans might be able to see in order to judge a persons emotional state.

The ability to see the skin on the face may be even more important for the face of women than the face of men, hence why women lack facial hair. In many primates, when a female is at her peak fertility, she has a red vaginal swelling that becomes more prominent at this time, so the males know that she is ready mate. In humans, the upright stance, combined in more recent millennia by the use of clothing, has made such swelling impractical and invisible; thus the only sign that a human female may be at peak fertility might be by the colour of the skin on her face, and the redness of her lips (possibly also changes in her breasts – an organ that is capable of swelling in the way that the vagina of other primates might swell).

Another hypothesis regarding the nature of human hair looks at the fact that humans are one of the very few apes that are comfortable at swimming, and it was conjectured that early in the development of humans, humans inhabited some coastal strip or beach area (or the banks of rivers) that meant they needed to be efficient swimmers. The argument goes that having lots of hair on the body would make swimming more difficult, but since the head was mostly out of the water, hair on the head did not make much difference.

One of the arguments for the proponents of water births is that early human females may well have gone into the sea to give birth, and this is why some women find it more comfortable to give birth in a pool. Another argument in that has been proposed to favour this theory is that when human babies are born they are covered in a layer of protective fat, and the only other mammals that are covered in the external fat are sea living mammals, or those that enter the water very soon after birth, such as some seals.

If women did give births in water, it might also explain why their nipples are encased in large balloons of fatty tissue (i.e. their breasts), since these are buoyant in water, and if the woman, rather than swimming, was simply standing of sitting, chest high in water, the breasts would float on the surface, and make it easier for the newborn to be held to the breast without risk of drowning.

As for why hair on our head continually grows longer, this is actually not true. Hair, on all parts of the body, will grow to a certain length, will then fall out, and be replaced by new hair. This is also true for the hair on the head. What is different about the hair on the head is that for some people (and this varies from person to person, and from racial group to racial group), the hair on the head can grow to several feet in length before falling out. As I said, this varies between racial groups (as well as between individuals), and we don't know what the first humans looked like, so we don't know what is the natural length of the hair on the head (i.e. whether those people who cannot grow long hair may be more like our ancestors than those who can grow long hair).

One thing that is interesting is that certain religious/racial groups make a virtue of long hair. Sikhs never cut their hair, but bundle it under their turban, and similarly, traditional Jews allow at least some of the hair to grow long (look at the story of Sampson in the bible). Whether this is an indication that at some time in the past, the ancestors of these groups took long hair as a sign of superiority over their short haired cousins, and thus people in these communities with long hair had greater social status, and maybe these people had better reproductive success – maybe, maybe, maybe ... who knows.

As I said, lots of guesses (and I'm sure there are other ones I have not heard about), but no proof of anything.

In hot places having curly hair on the top of your head would be useful as insulation from the sun, and the longer it was the better it would be. Otherwise we get into the realms of sexual selection again.

young people tend to have less hair, which may have put a sexual selection pressure on women to be less hairy so to look younger and more fertile.

Long straight hair shows that you have been well fed for several years, which will indicate fertility in women, and competence in men, so this could have been another form of sexual selection. Although it would have had to og developed after things like combs and flint knives were developed to look after it. So it could even have been some sort of proto-fasion thing...